1. Field of the Invention
The present invention relates to a light emitting device as a constituent member of a large screen apparatus used in a stadium or the like.
2. Description of the Prior Art
FIG. 1(a) is an exploded perspective view of a conventional light emitting device disclosed in Japanese Patent Laid Open No. 100854/89 for example. In the same figure, the reference numeral 1 denotes a front panel on which are arranged fluorescent elements 2 in a matrix form and which covers one opening portion of a square frame-like spacer 3; the numeral 4 denotes a shielding electrode having openings 5 in corresponding relation to the fluorescent elements 2 arranged on the front panel 1; numeral 6 denotes a rear panel having cathodes 7 arranged thereon in corresponding relation to the fluorescent elements 2 to emit thermoelectrons for causing the fluorescent elements 2 arranged on the front panel 1 to emit light, the rear panel 6 covering the other opening portion of the spacer 3; numeral 8a denotes a first control electrode (scan electrode) for the cathodes 7; numeral 8b denotes a second control electrode (data electrode) for the cathode 7; numerals 9a and 9b denote wiring patterns for connecting the scan electrodes 8a and data electrodes 8b in common in the direction of row or column; and numeral 10 denotes an exhaust portion. Hereinafter, a space 3a surrounded by the spacer 3 will be designated the interior of the spacer, and each inside wall surface 3b will be referred to as the inner side face. In some case, the front panel 1 also serves as an anode. In the case where the front panel 1 does not serve as an anode, an anode is disposed between the front panel and the shielding electrode 4.
FIG. 2 is a wiring diagram showing wiring on the rear panel 6. In the same figure, S1 to S4 represent lead-out portions for the scan electrodes 8a connected in common in the row direction, while D1 to D4 represent lead-out portions for the data electrodes 8b connected in common in the column direction. FIG. 3 shows timings of signals applied to the scan electrodes 8a and data electrodes 8b. FIG. 4 shows a correlation between the arrangement of picture elements P11-P44 and the electrodes, and FIG. 5 explains the potential of each electrode and the flow of electron. Further, FIG. 6 shows an example of a display comprising a number of (two in the figure) light emitting devices A1, A2.
The operation of such a conventional light emitting device will be described below.
According to the basic principle of this type of a light emitting device, thermoelectrons emitted from the cathodes 7 are accelerated and strike against the fluorescent elements 2 arranged on the front panel 1, whereby the fluorescent elements 2 are excited and emit light.
Thermoelectron emitted from a cathode 7 behave as follows according to potential combinations of scan electrode 8a and data electrode 8b, as shown in FIG. 5.
1 In the case where both a scan electrode 8a connected in the row direction and a data electrode 8b connected in the column direction are positive relative to a cathode 7:
Thermoelectrons emitted from the cathode 7 by the positive potential of the data electrode 8b are deflected by the potential of the scan electrode 8a and reach an anode to cause a fluorescent element 2 to emit light.
2 In the case where the scan electrode 8a is positive and the data electrode 8b is negative:
The potential near the cathode 7 becomes negative under the negative potential of the data electrode 8b close to the cathode 7, whereby the emission of thermoelectrons is suppressed, so that the fluorescent element 2 does not emit light.
3 When the scan electrode 8a is negative and the data electrode 8b is positive, there are the following two cases.
a. In the case where an adjacent scan electrode 8a is positive, thermoelectrons emitted from the cathode 7 are deflected toward the adjacent scan electrode 8a by the negative potential of the scan electrode 8a in question, so the fluorescent element 2 does not emit light. PA1 b. In the case where the adjacent scan electrode 8a is also negative, although the potential of the data electrode 8b is positive, because of a small area of the data electrode, the potential in the vicinity of the cathode 7 becomes negative under the influence of the negative potential of both-side scan electrodes 8a, whereby the emission of thermoelectrons is suppressed and so the fluorescent element 2 does not emit light.
4 In the case of both scan electrode 8a and data electrode 8b being negative, the potential in the vicinity of the cathode 7 becomes negative, whereby the emission of thermoelectrons is suppressed and so the fluorescent element 2 does not emit light.
As a result, from the relation between the wiring illustrated in FIG. 2 and arrangement of fluorescent elements 2 in FIG. 4, the fluorescent element 2 positioned at an intersecting point of positive potential applied scan electrode 8a and data electrode 8b emits light. First, when a signal is applied to S1, P11 to P14 are selected and emit light in accordance with the potential of data electrodes 8b (D1 to D4). Next, when a signal is applied to S2, P21 to P24 are selected and emit light also in accordance with the potential of data electrodes 8b. Therefore, as shown in FIG. 3, any desired display can be obtained by successively applying scan signals to the scan electrodes 8a and optional data signals to the data electrodes 8b.
The following description is now provided about a sealing process for the conventional light emitting device.
First, in bonding the spacer 3 to the front panel 1 and also to the rear panel 6, as shown in FIG. 7, frit glass 12 is applied uniformly to each bonding surface of the spacer 3 by means of a dispenser 11, and bonding is effected through the frit glass (although the frit glass 12 itself is a powder, fluidity is imparted thereto by mixing it with a suitable solvent).
At the time bonding, the scan electrodes 8a and data electrodes 8b are drawn out from the spacer rear panel bonded portion to permit the transmission of signals between the light emitting device and an external device (not shown). In this way the sealing process is carried out.
FIG. 6 shows an example of a display comprising a number of light emitting devices A1, A2. It is seen from this figure that in order to make the joint portion between adjacent light emitting devices A1 and A2 inconspicuous, it is necessary to provide between adjacent light emitting elements 2 in each light emitting device a space T2 which is twice or more as large as a dead space (width T1) provided around the light emitting device.
FIG. 8 shows an example in which cathodes 7, etc. are provided on a ceramic substrate 13, not on the rear panel 6. In this case, scan electrodes 8a and data electrodes 8b are drawn out to the exterior through both the ceramic substrate 13 and the rear panel 6. The numeral 14 denotes a shielding electrode.
Since the conventional light emitting device is constructed as above, when frit glass is applied uniformly onto each bonding surface of the spacer 3, it is necessary that the amount of frit glass discharged from the dispenser nozzle and the moving speed of the dispenser be always kept constant. However, this is difficult particularly at the corner portions, thus sometimes resulting in that the amount of frit glass applied is not uniform in some points. Consequently, as shown in FIG. 9, there may occur protrusion of frit glass, or as shown in FIGS. 10 and 11, there may occur a positional deviation, or displacement, between the spacer 3 and the front panel 1 and also between the spacer and the rear panel 6 (imbalance in pressure against the panels may be another cause of such displacement). Therefore, it is necessary to grind the protruded portion (the grinding may cause fine flaws, resulting in deterioration in strength of the glass). There may arise further problems such as deterioration of the mechanical accuracy and variations in luminance. The openings of the shielding electrode 4 which emit electrons are influenced by static electricity of the inner side faces of the spacer 3. Since the inner side faces of the spacer 3 are positively charged, if the openings of the shielding electrode 4 approach the spacer 3 due to displacement of the rear panel 6, the openings are strongly influenced by the positive potential of the inner side faces of the spacer 3, whereby the emission of electrons is accelerated. As a result, the luminance of the corresponding fluorescent element increases. On the other hand, as the said openings go away from the spacer 3, the luminance decreases. Thus, in the interior of the light emitting device there occur variations in luminance.
In the case where the scan electrodes 8a and data electrodes 8b are drawn out to the exterior through the ceramic substrate 13 and the rear panel 6, as shown in FIG. 8, a stress is induced in the ceramic substrate 13 due to the difference in thermal expansion coefficient among the ceramic substrate 13, rear panel 6, scan electrodes 8a and data electrodes 8b, resulting in cracking of the ceramic substrate.